1. Field of the Invention
The present invention relates to a medical stent.
2. Description of Related Art
Conventionally, a medical stent (hereinafter also abbreviated as ‘stent’) is placed to a stricture in a lumen inside a living body such as a blood vessel, a digestive tract, a bile duct, a pancreatic duct, and a urinary duct, in order to expand this stricture and maintain an open state.
A stent used in a bile duct such as that shown in Japanese Unexamined Patent Application, First Publication No. 2006-87712 is substantially tubular. Flaps which open in a natural state and can deform so as to close when a predetermined external force is applied, are provided on the distal-end side and the proximal-end side of this stent. The flaps engage with the entrance to the duodenal papilla and the end of the stricture of the bile duct, thereby preventing the stent from moving to the stricture.
Characteristics demanded of this type of stent include easy of bending in order to follow the curving shape of the bile duct in the body and the motion of the body, i.e. flexibility, and, at the same time, hardness (lumen-maintaining properties) so that it does not collapse when bent and can maintain the size of its own lumen.
One of this stent made from a soft resin (e.g. soft polyethylene, polystyrene elastomer, polyamide elastomer, polyester elastomer, polyurethane elastomer, etc.) formed in a tubular shape. While this can enhance the ease of bending of the stent, the thickness of the stent must be increased to prevent it from collapsing in the radial direction.
Another of the stent made from soft resin contains a blade which metal wire is woven like a mesh. In this case, when the stent is bent, the rigidity of the blade maintains the size of the lumen.
The stent is also required viewable under radioscopy. This is because the position of the stent needs to be confirmed by irradiation of X-rays when placing the stent or after placing it. It is important that both ends of the stent are astride the stricture to enable bile to reliably pass. And it is important that the flaps on the distal-end side of the stent reliably over the stricture to prevent the stent from moving (migrating, escaping) after it is placed. That is, it is necessary to confirm that the flaps are clearly open above the stricture and engaging with the stricture. When the stent has migrating into the bile duct, it must be removed while confirming its position under radioscopy. The proximal end of the stent is held with holding forceps or the like, whereby the stent is removed and collected. Therefore, in removing the stent, it is necessary that the whole stent can be seen under radioscopy, and that the proximal-end part of the stent can be confirmed clearly.
In placing the stent, there are cases where a plurality of stents are placed. In that case, it is important in this procedure to know the position of the first stent that was placed. Preferably, it is possible to see the whole stent, or to see its part other than both ends. This is to confirm whether the first stent is moving during subsequent stents procedures.
Japanese Unexamined Patent Application, First Publication No. 2004-147700 proposes a method of using a ring-like member or a plate-like member rounded to a substantially cylindrical shape to partially increase radioscopic visibility.
Japanese Unexamined Patent Application, First Publication No. H05-192389 proposes a medical stent made from an inner-layer material, a reinforcing layer, and an outer-layer material, with flaps formed only from the outer-layer.